This invention relates to gas burner control systems, and particularly to an improved gas burner control system for gas fired furnaces of the induced draft or forced combustion types.
An induced draft furnace, sometimes referred to as a powered vent furnace, utilizes a small blower connected in an exhaust passageway of the combustion chamber for drawing air into the combustion chamber to provide a combustible air-gas mixture to the burner, and for discharging the products of combustion through the flue to the atmosphere. A forced combustion furnace utilizes a small blower connected in an intake passageway to the combustion chamber for drawing in air and forcing it into the combustion chamber to provide a combustible air-gas mixture to the burner, and for forcing the products of combustion out through the flue to the atmosphere. In either type, the air drawn in may be ambient air surrounding the furnace or outside air provided through a conduit connected between the furnace and air outside the dwelling.
In either furnace type, it has been conventional, for safety reasons, to provide control means for preventing or discontinuing burner operation in the event of a blower failure or a blockage in the intake air passageway or the exhaust flue. While such control means have taken many forms, the most commonly used control means appears to be devices of the pressure sensitive type. Such pressure sensitive control devices are constructed and arranged to sense pressure or pressure variations in the fluid path controlled by the blower, and to prevent or discontinue burner operation when the sensed pressures are outside predetermined values.
Such pressure sensitive control devices typically include a diaphragm and cooperative mechanically operated electrical switch. The device is responsive to pressure on the diaphragm, and as such, cannot distinguish between variations in pressure caused by actual changes in fluid flow velocity due to, for example, a partially blocked flue, from variations in pressure caused by, for example, a change in the density of the fluid in the fluid-flow path. Also, such a device must allow for various tolerances which will affect operation, such as tolerances in the differential of the mechanically operated electrical switch. Therefore, when such a device is calibrated, the above factors are considered, and the sensitivity of the device to degrees of blockage in the fluid flow path is compromised. Also, in such a device, one side of the diaphragm is connected by a hose to the area in which the pressure to be measured exists. In some applications, such as in some furnaces of high operating efficiency, wherein water condensate is produced, there is a tendency for such condensate to settle in the hose. Such water condensate in the hose is undesirable since it changes the operating pressures of the device. While this problem can be alleviated by proper location of the device or by providing an air vent in the hose, it is believed more desirable to provide a device which is not susceptible to such water condensate and thus can be mounted in the most convenient and economical location.